An opening or gap is purposely provided between adjacent concrete structures for accommodating dimensional changes within the gap occurring as expansion and contraction due to temperature changes, shortening and creep of the concrete caused by prestressing, seismic cycling and vibration deflections caused by live loads, and longitudinal forces caused by vehicular traffic. An expansion joint system is conventionally installed in the gap to provide a bridge across the gap and to accommodate the movements in the vicinity of the gap.
Bridge and roadway constructions are especially subject to relative movement in response to the occurrence of thermal changes, seismic events, and vehicle loads. This raises particular problems, because the movements occurring during such events are not predictable either with respect to the magnitude of the movements or with respect to the velocity of the movements. In some instances bridges have become unusable for significant periods of time, due to the fact that traffic cannot travel across damaged expansion joints.
Modular expansion joint systems typically employ a plurality of spaced-apart, load bearing members or “centerbeams” extending transversely relative to the direction of vehicle traffic. The top surfaces of the load bearing members are engaged by the vehicle tires. Elastomeric seals extend between the load bearing members adjacent the tops of the load bearing members to fill the spaces between the load bearing members. These seals are flexible are therefore stretch and contract in response to movement of the load bearing members. A plurality of elongated, longitudinal support members are positioned below the transverse load bearing members spanning the expansion gap between the roadway sections. The elongated support members support the transverse load bearing members. Each end of the support members is received in a housing embedded in the roadway sections.
In single support bar (SSB) modular expansion joint systems, a single support member is connected to all the transverse load bearing members. The load bearing member connection to the single support bar member commonly consists of a yoke. The yoked connection of the single support bar member to a plurality of transverse load bearing members provides a sliding or pivoting connection in the SSB modular expansion joint systems. In a multiple support bar (MSB) modular expansion joint system, each transverse vehicular load bearing member (ie, each “centerbeam”) is connected to a single longitudinal support bar member.
In MSB systems, the friction forces for the left edge beam and right edge beam oppose each other. If the forces are close or equal in magnitude, then they essentially cancel each other out. The spring forces govern, qualitatively the MBS system can be approximated as a series arrangement of spring.
In SSB systems, the SSB centerbeam virtually always experiences yoke friction resisting movement towards equilibrium and has no neutralizing friction force as in the MSB system. SSB systems rely on traffic vibration to dynamically “shake down” strain energy in the springs to restore equilibrium (referred to as stagnation zone movement. Accordingly, SSB systems often display a fanning type equidistance, where the first cell on the active side opens the greatest, the second a less than the first, the third less than the second, etc . . . .
Because of friction force differences, SSB systems and MSB systems using equidistance springs respond differently. SSB systems perform well in slow movements applications, for example bridge structure thermal movements. MSB systems are inherently better suited to accommodate faster movements, such as bridge superstructure flexure due to changes in vehicular loading position.
MSB systems are subject to size constraints. A design point is reached where the use of multiple support bars take up too much room and will not fit on the structure. Hence large structures often use SSB designs, but they do not perform as well as MSB systems in high speed environments.